1. Field of the Invention
The present invention relates to electroacoustic transducers and to acoustic galvanic isolation devices.
2. Discussion of the Related Art
An electroacoustic transducer converts an A.C. electric signal into acoustic waves (transmission) or conversely (reception). By associating electroacoustic transducers respectively operating in transmit and in receive mode on either side of a substrate, a galvanic isolation can be achieved.
FIG. 1 illustrates a device of galvanic isolation by acoustic connection described in unpublished French patent application 09/58864 filed on Dec. 11, 2009.
This device comprises a substrate 10 made of any material capable of transmitting acoustic waves, for example, a glass or silicon wafer. As an example, on a first surface of a silicon substrate 10, a first layer 12A of a heavily-insulating material, for example, thermal silicon oxide or another thermal oxide, extends. On the first layer of insulating material 12A is formed of a first array 14A of ultrasonic transducers connected in parallel. These transducers comprise a conductive layer 16A formed on insulating layer 12A and forming a first electrode common to all transducers. Conductive layer 16A may be made of heavily-doped polysilicon or of a metal.
Above conductive layer 16A is formed a layer 17A of a dielectric material, for example, silicon nitride. Membranes 18A are defined in layer 17A above cavities 20A.
On membranes 18A and opposite to cavities 20A are formed second electrodes 22A. These electrodes may be made of aluminum.
One or several contacts 24A are formed on first electrode 16A. Electrodes 22A are connected to a node 26A.
Symmetrically, the second surface of substrate 10 comprises elements 12B, 14B, 16B, 17B, 18B, 20B, 22B, 24B and 26B homologous to elements 12A, 14A, 16A, 17A, 18A, 20A, 22A, 24A and 26A.
The device of FIG. 1 operates as follows. An A.C. input signal which is desired to be transmitted from the electroacoustic transducers of array 14A to the electroacoustic transducers of array 14B and a bias voltage are applied between contact 24A and 26A of first array 14A of transducers. The A.C. input voltage causes the oscillation of the different membranes 18A of the transducers of first array 14A.
Substrate 10 propagates the ultrasound acoustic waves created by the oscillation of membranes 18A towards second array 14B. Advantageously, in the case of a silicon substrate 10, the path of the acoustic waves is highly directional, which enables the waves to be well received at the level of second array 14B.
The acoustic waves transmitted by substrate 10 reach the second array of ultrasonic transducers 14B, which causes the vibration of membranes 18B. A D.C. voltage generator is placed between contact terminals 24B and 26B of the second array. The vibration of membranes 18B then causes a variation of the voltage across an electric output circuit connected to contact 26B.
Such an isolation may, for example, be used to isolate a control circuit referenced to ground from the control terminals of a power circuit referenced to a higher voltage, for example, the mains.